The semiconductor industry has experienced rapid growths due to continuous improvements in the integration density of semiconductor devices (e.g., transistors, diodes, resistors, capacitors, etc.) and the continuous reduction in the critical dimension (CD) of semiconductor devices. With the continuous reduction of the CD of semiconductor devices, the scales of the gate, source and drain of a transistor decrease accordingly.
The gate of a transistor is the area where the critical on-off switching of the transistor takes place. Historically, doped polysilicon has been used as the gate electrode of CMOS transistors. When the gate dimension in a modern transistor is so thin, a gate leakage may occur through a process called “quantum mechanical tunneling.” New generations of transistors may be made using metals to replace polysilicon gate electrodes. A metal gate enables a faster, and a more reliable performance for a transistor, with reduced gate leakage. These new metal gates may reduce the NMOS transistor gate leakage by more than 25 times and PMOS transistor gate leakage by more than 1000 times while simultaneously delivering improved drive current and improved circuit performance.
While the metal gate reduces the gate leakage for CMOS transistors, it can have a large resistance, which can lead to a low yield for the transistor fabrication process. New methods and apparatus are needed to reduce the resistance of a metal gate for the transistors.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.